Types of Radiation & Retention:
External Beam Radiation Therapy: The machine sends beams through your body; once it's off, no radiation remains in you.
Diagnostic Imaging (X-rays, CTs, MRIs): No radiation stays in your body at all after the scan.
Nuclear Medicine Imaging: A tracer is used, but it's eliminated by the body within days to weeks, depending on the isotope.
Internal Radiation Therapy (Brachytherapy):
Temporary Implants: Sources are placed for minutes to days and removed, making you radiation-free afterward.
Permanent Implants (e.g., Seeds): These stay in the body forever but become non-radioactive after several weeks or months as their radiation "burns out," with little to no risk to others.
Systemic Radiation (Radioactive Drugs): Drugs travel through the body and are excreted, though they can make you radioactive for a short period.

Responding to a comment above that radiation stays in your body.